Silent Hunter allows you to crew a submarine with several people. Most intense stuff you'll ever play. Requires some learning though - it's no fun with all automatic aids. You'll want to make fire solutions manually, estimate distance to target and identifcation by silhuette, listen for contacts via audio etc), so it's proabably not suitable for a group of casuals. But it's well worth the work. There are 4 games in the series, I know at least one allows it but I don't remember which. You'll have to figure that part out yourself.